Previously we purified the specific cell surface receptors for both transferrin, the iron transport protein, and transcobalamin II, the cobalamin (vitamin B12) transport protein. Over the past year, we have undertaken studies measuring iron and cobalamin uptake by cells in both normal and abnormal conditions. Very recently, we have been successful in iodinating the transferrin receptor protein and have developed a double antibody radioimmunoassay which measures as little as 5 ng of receptor protein. Experiments have been initiated to study hemochromatosis, a genetic disease causing iron overload. These studies have included measuring iron uptake by hemochromatotic cells in tissue culture, and measuring the properties of purified hemochromatotic transferrin as compared to normal subject transferrin. Other iron uptake experiments utilize a rat breast tumor model in which in vivo experiments have shown that injected radioactive iron bound to transferrin is taken up by the tumor in increasing amounts for time periods up to 48 hrs after injection. We have also studied a family in which there is the presence of an abnormal transcobalamin II protein which has an absent or markedly decrease ability to bind cobalamin. This defect results in diminished cobalamin uptake by cells. Using an immunofluorescent assay for the transferrin receptor, we have developed an automated reticulocyte assay using a fluorescent detector. Since a higher density of transferrin receptors are found on certain tumor cells as opposed to normal cells, we plan to initiate studies using a fluorescent cell sorter which may distinguish circulating neoplastic cells in patients blood.